Security is an important concern in the realm of documents and digital production and/or reproduction of documents. Known digital image printing/copying systems produce documents of such high quality that a need has been identified to prevent effective printing/copying of certain documents such as high-value printed items including tickets, financial instruments, security passes, and the like. Known techniques include printing the original document in a manner such that it includes a digital “watermark” using only conventional paper and toner or ink. A digital watermark is defined as information, for example one or more letters, words, symbols or patterns, that is at least partially (preferably fully or at least substantially) hidden in a printed image under normal viewing conditions but that is more clearly discernable under certain specialized viewing conditions. Unauthorized reproduction of documents including such digital watermarks typically degrades or obscures the digital watermark, which can aid in detection of counterfeit documents.
A fluorescence mark (also sometimes referred to as a “UV mark”) is one example of a known digital watermark. Methods and systems are known for including fluorescence marks in printed documents using conventional papers (e.g., ordinary “copy paper” or “printer paper”) and ordinary inks/toners (e.g., CMYK ink/toner), specifically by using metameric colorant mixtures. Under visible lighting conditions (e.g., electromagnetic radiation wavelengths of about 400-700 nanometers (nm), the different colorant mixtures that are printed on respective adjacent portions of the paper together define an overall printed document region that appears substantially uniform in color. Under ultraviolet (UV) lighting (e.g., electromagnetic radiation wavelengths shorter than about 400 nm), these different colorant mixtures exhibit different UV absorption and, thus, different suppression of UV fluorescence of the optical brightening agents used in conventional printing/copying papers such that the region printed with the colorant mixture that suppresses less of the substrate fluorescence appears as a lighter/brighter region while the adjacent area printed with the colorant mixture that strongly suppresses substrate fluorescence appears as a darker region. These contrast variations under UV lighting are used to create watermark patterns, e.g., numbers, letters, symbols, shapes.
An example of this is shown in FIG. 1, wherein a colorant mixture “B” is selected and applied to patch area BP which, in this example, is shaped as the alphanumeric symbol “0”. Further, a colorant mixture “A” is selected and applied to patch area AP arranged in substantially close spatial proximity to patch area BP, and thereby providing a background around/adjacent patch area BP. Both colorant mixture A and colorant mixture B are comprised of a suitably selected colorant or colorant mixtures, but colorant mixtures A and B are different mixtures. Each colorant mixture A or B may be, for example, either a single CMYK colorant or any mixture of CMYK colorants. In the illustrated example, colorant mixture A will be selected so as to provide higher UV absorption (greater substrate fluorescence suppression) than that selected for colorant mixture B. The colorant mixtures A and B will also be selected to match each other closely in their average color and luminance when viewed under visible light conditions. As shown at UV in FIG. 1, under UV lighting conditions, patch BP will appear brighter as compared to patch AP, due to the relatively limited suppression of the fluorescence of the optical brightening agents in the paper substrate as compared to the patch AP, thus forming a watermark W1. In contrast, under visible light conditions as shown at VIS, patches AP,BP are at least substantially indistinguishable. By way of example, an approximate 50% grayscale gray colorant mixture may be realized with a halftone of black (K) colorant only and used for colorant mixture B to print patch BP. This may then be color-matched against a colorant mixture comprising yellow (Y) colorant mixed with enough cyan (C) and magenta (M) to yield a similar approximate 50% grayscale gray colorant mixture A which is used to print the patch AP. With the given high area coverage of colorant mixture A, this colorant mixture will provide much higher absorption of UV or suppression of native substrate fluorescence as compared to the patch BP, so that under UV lighting conditions, the patch BP will be readily apparent as a watermark W1. The two colorant mixtures will appear quite nearly identical “gray” under normal visible light viewing as shown at VIS in FIG. 1. Thus, when a document including such a fluorescence mark is subjected to UV illumination, the watermark W1 is revealed. A printed “look-alike” document or mere photocopy will not include the watermark.
It should be noted that here and in the following, we use the terms “suppress” or “selectively suppress” to indicate the property of the first colorant mixture as having a higher UV suppression than the corresponding second colorant mixture.
Additional details and variations relating to fluorescence marks are disclosed in U.S. patent application Ser. No. 11/382,897 filed May 11, 2006 in the name of Raja Bala and Reiner Eschbach and entitled “Substrate Fluorescence Mask for Embedding Information in Printed Documents” and U.S. patent application Ser. No. 11/382,869 filed May 11, 2006 in the name of Raja Bala and Reiner Eschbach and entitled “Substrate Fluorescence Pattern Mask for Embedding Information in Printed Documents” and the disclosures of both these applications are hereby expressly incorporated by reference into the present specification.
An important related aspect of the above is the ability to decode (identify and/or extract) fluorescence marks from a printed document. In a conventional method, the relevant portion of the printed document is illuminated using a UV light source so that a human observer or machine can seek to discern the presence of the fluorescence mark and confirm that the fluorescence mark matches the required/expected security mark such as a known alphanumeric code, a pattern, symbol, design or the like. A drawback of this approach is that a UV light source is required, and most conventional printed document scanners (such as those commonly used in offices to scan and store or reproduce documents) do not include the required UV light source. Also, a drawback of many prior methods is the need to use a human observer to view and decode the fluorescence mark and compare the mark to a required security mark.